I'm not saying they're useless, they're an educated guess, a valuable means of communicating an expert opinion. It can be replaced with "I have high confidence of success".

The assessment is subjective, includes personal beliefs, assumptions, etc. not just measurable objective points. The measurement uncertainty at every step and the confidence interval are disappointing despite people using percentages, suggesting confidence to the percent. A scale from 1 to 5 is most of the times just as accurate.

2 people looking at the same situation could have very different degrees of belief, which is why 2 engineers could give very different estimates. But if I say I believe the likelihood of something is 73.8% I might be more credible to the listener. Looks like I calculated, not like the guy who said "3 out of 5 we get it done".